The nation's largest long-distance carrier said it will study ways to restructure its rates, focusing on basic rate consumers who make few calls. The company will freeze its current per-minute basic rates until it has completed its review and has informed customers of any changes and available options.

The move came hours after telecommunications regulators said they would enforce a pledge by AT&T to pass on to consumers billions of dollars in savings from a restructuring of the phone system. Last week, the Federal Com-munications Commission announced it would cut by $3.2 billion the ''access fees'' local phone companies charge long-distance carriers to connect calls costs that phone users typically pay.

In turn, AT&T and Sprint pledged to pass along these reductions to consumers. But AT&T said it would increase its per-minute rate for basic schedule customers Monday through Saturday.

Consumer advocates, skeptical about the promises by the companies and the FCC, warned against declaring victory because the long-distance carrier could decide to raise rates in the future.

''This reflects their embarrassment at the public outrage, but what happens tomorrow or next week or next month when no one is looking?'' said Gene Kimmelman, co-director of Consumers Union's Washington office.

''Unfortunately, the FCC failed to develop a compliance system with verification. AT&T will be in a position in the dead of night to raise rates again.''

The companies ''have learned over the years that if you change two numbers at one time, the FCC gets confused,'' said Mark Cooper, research director for Consumer Federation of America. He estimated that with the increases, AT&T basic rate consumers with more than 12 minutes of calls a month would have had higher bills.

FCC Chairman William Kennard said Wednesday the agency will hold AT&T to its word to provide consumers with rate reductions and to tell them which plan would be most cost effective.

Following AT&T's announcement, Kennard said he was glad the company had deferred its plans.

''The commission significantly reduced access charges last week to benefit consumers. We must ensure that consumers actually realize tangible benefits. If there is any doubt about this, it must be cleared up quickly,'' Kennard said.

The company said it will go ahead with eliminating its current $3 monthly minimum fee for old and new customers, but would not put off increasing its basic rates.

''Our goal is to give customers the great service at a great price,'' said Robert Aquilina, senior vice president of AT&T Consumer Services. ''Customers who make few calls can get the best prices by scheduling their calls when rates are lowest.''

AT&T had planned to make the following changes to its rate:

Under the old basic rate, the company charged 26 cents per minute during weekday peak times, 7 a.m. to 7 p.m., and 16 cents per minute during off-peak times, 7 p.m. to 7 a.m. Now, customers on the basic schedule will pay 29 cents a minute all day Monday through Friday.

Under the old basic rate, the company charged 11.5 cents a minute on Saturday and Sunday. Now, customers will pay 29 cents a minute on Saturday, with rates dropping to 7 cents a minute on Sunday.

The company raised from $3 to $5 the monthly fee that comes with its one-rate plan, in which all calls cost 15 cents a minute.

It was clear the FCC plan announced last week would bring the most immediate benefits to those who make few or no long-distance calls, because AT&T has pledged to do away with a minimum $3 monthly fee for both old and new callers.

So people who made no calls a month would see their bills drop from $9.39 to $4.68 a 50 percent reduction. But the impact on moderate or heavy users depends on how much the carriers reduce their per-minute rates.

Sprint said it is still determining how it will pass on benefits to consumers.